(1) Field of the Invention
The present invention relates to optoelectric articles and a process for producing the same.
(2) Related Art Statement
The lithium niobate (LiNbO.sub.3) single crystals and the lithium tantalate (LiTaO.sub.3) single crystals have been expected as materials for optoelectronics. It is known that a thin film of lithium niobate may be formed on a substrate made of a single crystal of lithium niobate or the like by the liquid phase epitaxial process.
For example, according to the description in "Appln. Phys. Letters", vol. 26, No. 1 (1975), pp 8-10, a thin film of a single crystal of lithium niobate having an almost stoichiometric composition (Li/Nb=1) is formed on a substrate of a single crystal of lithium tantalate by the liquid phase epitaxial process. According to "J. Appln. Phys.", Vol. 70, No. 5 (1991), pp 2536-2541, a thin film of a single crystal of lithium niobate is formed on a substrate of lithium niobate doped with 5 mol % magnesium oxide by the liquid phase epitaxial process, while the ratio of Li.sub.2 O/Nb.sub.2 O.sub.5 of a melt is varied and the lattice constant is varied. According to "J. Cryst. Growth", Vol. 132 (1993), pp 48-60, a thin film of a single crystal of lithium niobate having an almost stoichiometric composition is formed on a substrate of lithium niobate doped with 5 mol % magnesium oxide by the liquid epitaxial process. In unexamined Japanese patent application Laid-open No. 5-117096, a thin film of a single crystal of lithium niobate having a composition of Li/Nb=1 is formed by the liquid phase epitaxial process.
Single crystal substrates made of lithium niobate, which have been widely used in optical devices, etc., are now produced by the pull-up process (Czochralski process: CZ process), and their compositions have a congruent composition of Li/Nb=48.6/51.4. Single crystals of lithium niobate having compositions other than the congruent composition Li/Nb=48.6/51.4 are difficult to produce by the pull-up process, and crystalline substrates excellent enough to withstand optical applications have not been produced.
However, since the lattice constant of the single crystal films of lithium niobate formed on the single crystal substrates by the liquid phase epitaxial process differs from that of the substrate having the congruent composition, lattice mismatch occurs at the interface between the film and the substrate. Accordingly, it is difficult to form a thin film having excellent crystallinity on the substrate.
Further, the single crystals of lithium niobate have been used as substrates for optical devices. In this case, an optical waveguide is formed in the substrate by, for example, a proton-exchanging process. In such a proton-exchanging process, lithium atoms of the lithium niobate single crystal film are partially replaced by hydrogen atoms to form a layer made of H.sub.x Li-nNbO.sub.3 and having a high refractive index according to the following reaction: LiNbO.sub.3 +xH.fwdarw.H.sub.x Li-xNbO.sub.3 +xLi.
The present inventors formed a single crystal film of lithium niobate on a single crystal substrate by the liquid phase epitaxial process, and formed a proton-exchanged layer by using the resulting substrate. Then, the inventors examined the crystallinity of the proton-exchanged layer in detail. As a result, it was clarified that the crystallinity of the layer was deteriorated after replacement with the protons. It is believed that since lithium atoms are removed from the crystalline lattice of the single crystal film of lithium niobate, and instead replaced by hydrogen atoms, the crystalline lattice is distored during this step.